How to restore visibility of animation panel?
Every portion of the window can be configurated rappresenting an editor, the animation-related windows are: timeline, graph editor, dope-sheet, and NLA editor.



You can create a new portion of window by selecting and dragging the little triangle in the up right angle of every portion of window. Right clic on the separation line between two windows to join them.

Check also the upper header (info window), for default templates of windows organization.



If you have the option "back to previous", click it!